1. Field of the Invention
This invention relates to a method for treating a subterranean formation penetrated by a well, and more particularly concerns a method for acidizing relatively high temperature subterranean formations penetrated by a well.
2. Description of the Prior Art
Acidization of wells is a well-known process for increasing or restoring the permeability of subterranean formations to thereby facilitate the flow of formation fluids, such as oil, gas or a geothermal fluid, into the well from the formation, and also facilitate the injection of fluids through the well into the formation. Acidization involves treating the formation with an acid, typically hydrochloric acid, in order to dissolve clogging deposits, such as carbonate scale, thereby opening pores and other flow channels and increasing the permeability of the formation. Hydrofluoric acid or a mixture of hydrofluoric and hydrochloric acids, commonly known as "mud acid", is typically employed to dissolve siliceous deposits.
Numerous acidization methods have been proposed to cope with varying well conditions and special formation problems. However, in recent years the increased activity in drilling very deep oil and gas wells and geothermal wells has outpaced the development of suitable acidization methods, primarily due to the high temperature of these formations.
A problem common to all the prior art acidization processes is the corrosion of the well equipment, particularly the downhole tubing and casing, which is exposed to the acidizing fluid. Because the reactivity of an acid is significantly increased at higher temperatures, the corrosion of well equipment is especially serious in the acidization of high temperature formations.
Corrosion inhibitors are generally incorporated into the acidizing fluid prior to its injection into the well. However, the effectiveness of the known corrosion inhibitors decreases at higher temperatures, and the expense of the corrosion inhibitors, which are significant even at low temperatures, become prohibitive at temperatures above about 250.degree. F. Another difficulty with the known corrosion inhibitors, especially when used in the large quantities required in high temperature wells, is their tendency to form insoluble solids upon reaction with formation materials, thereby damaging the formation.
Another problem in the acidization of high temperature formations is that the acid is rapidly consumed by the reactive material immediately adjacent the borehole before the acid can penetrate any significant distance into the formation. Without adequate formation penetration, the acidization operation is of little value. In view of these problems, the prior art acidization methods are limited, as a practical matter, to the acidization of formations having temperatures on the order of 250.degree. F. and less.
The aforementioned problems have been overcome to a large extent by the use of the methods disclosed in my U.S. Pat. No. 4,148,360 and my copending application Ser. No. 14,054 wherein a substantially anhydrous acid precursor is injected into the formation and is allowed to hydrolyze in situ to generate a hydrohalic acid. However the acid precursors used in these methods have relatively high liquid densities. For example, tetrachloromethane has a specific gravity of about 1.594. These high densities may result in various problems depending, inter alia, upon the particular injection sequence employed, the configuration of the well and the characteristics of the formation being acidized. For example, where alternate slugs of an aqueous liquid and the acid precursor are injected, the acid precursor may segregate by gravity through the aqueous liquid slug preceeding it. Under other circumstances, the acid precursor may segregate by gravity to the lower zones of the formation to be acidized, which segregation could result in an undesirably nonuniform acidization of the formation zones. Also, a portion of the dense acid precursor could settle into the bottom of a well during injection, and hydrolysis of that portion could result in the undesirable corrosion of well equipment. Thus, a need exists for a method for acidizing high temperature subterranean formations using acid precursor-containing treating fluids having a lower density than the acid precursor.
Accordingly, it is a primary object of this invention to provide a method for acidizing high temperature subterranean formations using treating fluids having intermediate to relatively low densities.
Another object of the invention is to provide an acidization method which results in no more than an acceptable rate of corrosion of metal well equipment.
Still another object of the invention is to provide a simple but effective method for acidizing subterranean formations having temperatures on the order of 250.degree. F. to 700.degree. F. and higher, which method results in little or no corrosion of the well hardware.
Yet another object of the invention is to provide an acidization method for high temperature formations which does not require the use of corrosion inhibitors or other expensive chemical additives.
A further object of this invention is to provide an acidization method in which a noncorrosive, nonscaling, acid precursor is displaced through a well and into a high temperature formation, wherein the precursor reacts in suit to generate a strong acid.
A still further object of this invention is to provide a method for acidizing those portions of high temperature formations which are relatively remote from a borehole.
Another object of this invention is to provide a method for simultaneously acidizing and hydraulically fracturing a high temperature subterranean formation.
Still further objects, advantages and features of the invention will become apparent to those skilled in the art from the following description.